The present invention relates to generally an electronic flash device and more particularly an electronic flash device capable of automatic flash duration adjustment and successive flashes.
Electronic flash device capable of successive flashes have been widely used with motor-driven cameras and in photographing a subject moving at a high velocity. There has been an increasing demand for electronic flashes capable of successive flashes at higher frequencies.
The prior art electronic flash devices capable of successive flashes is of the so-called series controlled type in which the flash duration is suitably adjusted by controlling a switching element connected in series to a flash lamp in response to a distance to subject, a film speed or an aperture. The frequency of the successive flashes with such flash devices will be discussed below.
In the series controlled type, in response to the exposure factors described above, the switching element connected in series to the flash lamp is controlled by a flash duration control circuit consisting of a commutation circuit consisting of a commutation capacitor, an auxiliary switching element and other associated circuit elements and a brightness sensor with a light sensor. Therefore, the frequency of successive flashes is in general dependent upon a frequency at which the flash duration control circuit can control the switching element. When this switching element is switched into the conduction state, the flash lamp is lighted but when it is switched into the nonconduction state, the flash lamp is turned off. Obviously, it is more difficult to drive the switching element into the nonconduction state than to switch it into the conduction state. As a result, some interval of time is needed after switching element has been switched into the nonconduction state before it can be switched again into the conduction state; that is, before the next flash is ready.
The discharge of the commutation capacitor causes the switching element to switch into the nonconduction state. As a result, the frequency of successive flashes is determined by an interval of time between the charging and recharging of the commutation capacitor; that is, by the charging time constant of the commutation capacitor. It follows, therefore, that the straightforward method for increasing the flash frequency is to select a short time constant. However, in the prior art electronic flash devices, a fixed charging resistance is connected in the circuit for charging the commutation capacitor without exception. It is possible, therefore, to make the charging time constant very short by selecting a charging resistor having a low resistance or a commutation capacitor having a low capacitance, but this gives rise to the problem that failures in switching the switching element into the nonconduction state or in extinguishing or deionizing the flash lamp tend to occur at high frequencies. As a result, the charging time constant cannot be made shorter than some limit. Thus, it has been so far impossible to attain the flash frequency of less than 0.1 sec. by mere efforts for making the charging time constant short without causing the failures or malfunctions described above.